XIX.] EXPERIMENT. 435 



of an air-pump, as did Du Sejour. It is equally impossi- 

 ble to prove that gravity occupies no time in transmission. 

 Laplace indeed ascertained that the velocity of propagation 

 of the influence was at least fifty million times greater than 

 that of light ; ^ but it does not really follow that it is in- 

 stantaneous ; and were there any means of detecting the 

 action of one star upon another exceedingly distant star, 

 we might possibly find an appreciable interval occupied in 

 the transmission of the gravitating, impulse. ' Newton 

 could not demonstrate the absence of all resistance to 

 matter moving through empty space ; but he ascertained by 

 an experiment with the pendulum (p. 443), that if such 

 resistance existed, it was in amount less than one five- 

 thousandth part of the external resistance of the air.^ 



A curious instance of false negative inference is fur- 

 nished by experiments on light. Euler rejected the cor- 

 puscular theory on the ground that particles of matter 

 moving with the immense velocity of light would possess 

 momentum, of which there was no evidence. Bennet had 

 attempted to detect the momentum of light by concentrat- 

 ing the rays of the sun upon a delicately balanced body. 

 Observing no result, it was considered to be proved that 

 light had no momentum. Mr. Crookes, however, ha.ving 

 suspended thin vanes, blacked on one side, in a nearly 

 vacuous globe, found that they move under the influence 

 of light. It is now allowed that this effect can be ex- 

 plained in accordance with the undulatory theory of light, 

 and the molecular theory of gases. It comes to this — that 

 Bennet failed to detect an efl'ect which he might have 

 detected with a better method of experimenting ; but if he 

 had found it, the phenomenon would have confirmed, not 

 the corpuscular theory of liglit, as was expected, but the 

 rival undulatory theory. The conclusion drawn from 

 Bennet's experiment was falsely drawn, but it was never- 

 theless true in matter. 



Many incidents in the history of science tend to show 

 that phenomena, which one generation has failed to dis- 

 cover, may become accurately known to a succeeding 

 generation. The compressibility of water which the 



' Laplace, System of the World, translated by Harte, vol. ii. ]i. 322. 

 - J'rmcipia, bL ii. sect. 6, Prop. xxxi. Motto'3 trauslution, vol. ii. 

 p. 108. 



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